1. Field of the Invention
This invention relates to methods of reducing the sulfuric acid contamination of waste water effluents from battery manufacturing operations by neutralization of the sulfuric acid with waste battery paste containing lead oxide.
2. Description of the Prior Art
The manufacture of so-called lead acid batteries produces two contaminating waste products which find their way into waste waters which are discharged into municipal sewerage systems and eventually into natural streams or bodies of water, thereby reducing the quality of the environment. These contaminants are sulfuric acid which is used to manufacture the battery electrolyte used in lead-acid batteries and also used in forming the battery paste which is pasted to the grids of the battery. The other contaminant is waste battery paste from the grid pasting operation, largely formed of lead sulfate and lead oxide. The paste is a solid waste which, after neutralization, is normally disposed of in solid waste dump areas. Many municipalities object to wastes containing lead for various reasons, the chief one of which is that there is a possibility of lead contamination of ground waters by percolation of normal rainfall through the solid waste disposal area, and when solid wastes are used for land fill operations, a high concentration of lead could present a health hazard to later occupants.
Previous procedures for neutralization of sulfuric acid involved the use of low-cost alkaline materials such as sodium hydroxide which, by reaction with the sulfuric acid, produces a sodium sulfate, which is then passed into effluent waste water streams and discharged to the local sewerage system. Some municipalities object to this disposal and technique because of reports that sodium sulfate would damage concrete sewer pipes. Moreover, since sodium sulfate is soluble in water, the sulfate ion content of the waste water is not decreased. Some agencies have set standards for sulfate ion content in waste water effluents, but any neutralization process which produces a soluble sulfate salt would present difficulties in meeting any standard.
Lime has also been employed as a neutralization agent for sulfuric acid and battery pastes which contain sulfuric acid. The disadvantage of lime neutralization is that it produces a solid sludge. When employed in paste neutralization the end product has no scrap value. Further, the combined lime sludge is normally contaminated with a small amount of lead, and for that reason is generally considered as an unacceptable waste product in solid waste disposal areas, especially those which are intended to be used for future commercial, industrial or residential sites, or those which border on bodies of water. It is assumed that the lead content of the calcium sulfate solid produced by lime neutralization could present a hazard to fish and other wildlife as well as to humans in the area.
Ammonia has also been used as an alkaline neutralizing agent, but this material presents problems from the point of view of producing soluble sulfates and a fume hazard, as well as a nitrogen pollutant in the waste effluent.
The process of the present invention eliminates many of the problems associated with the disposal of sulfuric acid and lead wastes which are produced by the lead-acid battery manufacturing operations. The process not only permits discharge of the aqueous component of the aqueous sulfuric acid waste at acceptable pH levels, i.e. approximately neutral, but also reduces the sulfate ion content of any discharge waste water to relatively low levels acceptable to most municipal sewerage and environmental control agencies. Furthermore, the process permits the recovery of lead in a form suitable for salvage or scrap and reduces the contamination of both solid and liquid wastes with lead compounds or lead ion.